1. Technical Field
The present invention is directed to lithographic processes for device fabrication and, in particular, to resolution enhancement techniques for such processes.
2. Art Background
In lithographic processes for device fabrication, radiation is typically projected onto a patterned mask (also referred to as a reticle) and the radiation transmitted through the mask is further transmitted onto an energy sensitive material formed on a substrate. Transmitting the radiation through a patterned mask patterns the radiation itself and an image of the pattern is introduced into the energy sensitive material when the energy sensitive resist material is exposed to the patterned radiation. The image is then developed in the energy sensitive resist material and transferred into the underlying substrate. An integrated circuit device is fabricated using a series of such exposures to pattern different layers of material formed on a semiconductor substrate.
An integrated circuit device consists of a very large number of individual devices and interconnections therefore. Configuration and dimensions vary among the individual devices. The pattern density, (i.e. the number of pattern features per unit area of the pattern) also varies. The patterns that define integrated circuit devices are therefore extremely complex and non-uniform.
As the complexity and density of the patterns increase, so does the need to increase the accuracy of the lithographic tools that are used to create the patterns. The accuracy of lithographic tools is described in terms of pattern resolution. The better the resolution, the closer the correspondence between the mask pattern and the pattern that is created by the tool. A number of techniques have been used to enhance the pattern resolution provided by lithographic tools. The most prevalent technique is the use of shorter wavelength radiation. However, this technique is no longer viable when exposure wavelengths are in the deep ultraviolet (e.g., 248 nm, 193 nm and 157 nm) range. Using wavelengths below 193 nm to improve resolution is not feasible because the materials used for lenses in optical lithography cameras absorb this shorter wavelength radiation.
Resolution enhancement techniques (RET) other than simply using shorter wavelength radiation have been proposed. These techniques use exotic illumination from the condenser (e.g. quadrupole illumination), pupil filters, phase masks, optical proximity correction, and combinations of these techniques to obtain greater resolution from an existing camera. However, such techniques typically improve resolution only for some of the individual features of a pattern. The features for which resolution is improved are identified as the critical features. The resolution of many other features is either not improved or actually degraded by such resolution enhancement techniques. Thus, current RETs require a compromise between resolution enhancement for the critical features and resolution degradation for the non-critical features. Such compromises usually require sub-optimal illumination of the critical features in order to avoid significant degradation in the illumination of the non-critical features.
Resolution enhancement techniques have been proposed to customize mask feature illumination in projection lithography for the various different features in the mask. One such technique is described in Matsumoto, K., et al., xe2x80x9cInnovative Image Formation: Coherency Controlled Imaging,xe2x80x9d SPIE, Vol. 2197, p. 844 (1994). That technique employs an additional mask and additional lens to customize the radiation incident on each feature of the mask. The arrangement proposed in Matsumoto et al. is illustrated schematically in FIG. 1. In the arrangement 10, light from an off-axis source 15 illuminates a first mask 20. Matsumoto describes the use of a quadrupole source 15 for off-axis illumination. In a quadrupole source 15 light passes from the source 11 and through a condenser lens 13 and an element 14 with four apertures 16 of identical size and configuration (FIG. 2) therein. The apertures 16 are spaced equidistantly from a common point (which is the optical axis of the optical projection camera). Furthermore, the centers of two apertures 16 and the common point are on a first line 17 and the centers of the other two sources and the common point are on a second line 18 perpendicular to the first line.
An image from the first mask 20 is projected through a first lens 22 onto a second mask 25. The first lens 22 is equipped with a pupil filter 23. The image from the second mask 25 is projected through a second lens 30 and onto a substrate 35 with a layer of energy sensitive material 40 formed thereon. A similar system is described in Kamon, K., xe2x80x9cProposal of a Next-Generation Super Resolution Technique,xe2x80x9d Jpn. J. Appl. Phys., Vol. 33, Part 1, No. 12B, p. 6848 (1994).
In the resolution enhancement techniques described in Matsumoto et al. and Kamon et al., the first mask has features that are identical to the features on the second mask. The features on the first mask diffract the radiation incident on the mask, and the diffracted radiation illuminates the identical feature on a second mask. For example, radiation transmitted through a grating pattern on the first mask is projected onto an identical grating pattern on the second mask. Similarly, radiation transmitted through an isolated line on the first mask is projected onto an identical isolated line on the second mask. When the diffracted energy from the first mask illuminates the identical feature on the second mask, the resulting image is often superior to an image obtained from quadrupole illumination of the pattern. Therefore, this resolution enhancement technique provides an improvement in aerial image quality (i.e. the image in the focal plane of the projection lens) over conventional off-axis illumination using the quadrupole system.
The above-described resolution enhancement technique provides customized illumination for more features than quadrupole illumination. However, the above-described technique does not improve the resolution of all features in the pattern. Furthermore, the two-mask system is costly and complex. Specifically, the system requires two precisely patterned masks instead of one. The corresponding features on the first and second masks must match precisely. The alignment of the first and second masks is also critical. Furthermore, the technique is limited because the features on the first mask are illuminated uniformly. Thus, the problems associated with non-customized illumination of a patterned mask are not eliminated by this system, but simply stepped further back into the optics of the system. Therefore, resolution enhancement techniques that improve the resolution of all features and are cheaper and easier to implement are sought.
The present invention is directed to a lithographic process for device fabrication that utilizes dark-field illumination for resolution enhancement of an image. As used herein, dark-field radiation is radiation from which the zero-order power is removed. The dark-field image is obtained using a mask in which the transmissive regions contain very fine patterns that are of a size selected so that the patterns will not be imaged (the non-imaged mask features hereinafter). In one embodiment of the present invention, the non-imaged mask features are combined with the pattern features in a single mask. In a second embodiment, the non-imaged mask features are formed on a first mask and the pattern features are formed on a second, separate mask.
In the process of the present invention, light from an off-axis source is incident on the mask that contains the non-imaged mask features. The off-axis angle (i.e. the angle between the off-axis illumination and the optical axis) is selected so that the zero order light transmitted through the mask is lost from the system. In the context of the present invention, light that is lost from the system is light that is not captured by the downstream system optics. This illumination is dark-field since a perfectly clear area on a mask illuminated in this manner would be black. Only radiation diffracted from the mask that contains the non-imaged features is captured by the downstream system optics.
The process of the present invention provides the flexibility of tailoring the non-imaged mask features to improve the illumination of all the mask pattern features, not just certain types of features. In the embodiment of the present invention wherein the mask pattern features and the non-imaged features are in a single mask, the non-imaged features are typically gratings that provide additional definition to the image. These are referred to as fine features and are typically placed within the confines of the pattern features on the mask. The mask pattern features are formed to modulate the radiation diffracted from the fine features. This modulation produces a secondary diffraction of rays that make a small angle around the beam diffracted from the fine pattern. The mask pattern features are coarser than the fine features. Consequently, light diffracted from the mask pattern features carries the pattern information and light diffracted from the fine features is the carrier that directs the light in the desired direction (i.e. into the optics downstream from the mask). The interaction between the side-band pattern information and the fine feature carrier introduces an interference pattern in the energy-sensitive resist material.
In the embodiment of the present invention wherein two masks are used, the first mask has opaque regions and clear regions that contain the non-imaged features. The incident light diffracted from the non-imaged features is projected through a lens and onto the second mask, which contains the mask pattern features. The zero order radiation from the condenser is not projected through the lens because the angle of incidence of the off-axis illumination is selected to ensure that the zero order light is not transmitted through the lens and onto the second mask. The use of a mask with sub-resolution size features allows for customization of the illumination of each feature of the second mask. For example, one sub-resolution feature is provided that transmits, at optimal angles, illumination to one feature (e.g. grating) on the second mask. A second sub-resolution feature is provided that transmits a cone of angles about the optic axis (often called top hat) to another feature. The sub-resolution features are tailored to provide customized illumination to pattern features on the second mask.